1. Technical Field
The present invention relates to a positioning method, a program of the positioning method, and a positioning device.
2. Related Art
As a positioning system using a positioning signal, a global positioning system (GPS) is widely known, and is used as a positioning device incorporated in, for example, a portable phone or a vehicle navigation device. In GPS, the present location of the positioning device is calculated by obtaining four parameters, namely three-dimensional coordinates representing the location of the device itself and a clock error, based on information such as locations of a plurality of GPS satellites and respective pseudo-ranges between the GPS satellites and the device itself.
A GPS satellite signal transmitted from a GPS satellite is modulated with a spread code called a PRN code unique to each GPS satellite. Further, it is known that the polarity of the PRN code can be reversed at a 20 millisecond interval by a phase modulation with the navigation data (see e.g., JP-A-11-258326).
In a positioning device of the related art, in order to acquire (extract) the GPS satellite signal out of a weak received signal, there is generally used a method of cumulatively adding (accumulating) the received signal for a predetermined cumulative addition period and correlating the signal resulted from the cumulative addition with a replica code of the PRN code.
However, as described above, the timing at which the polarity of the PRN code is reversed (hereinafter referred to as “polarity reversal timing”) can occur at a timing of a 20 millisecond interval (hereinafter referred to as “polarity reversible timing”). Therefore, in the case in which the cumulative addition (cumulative calculation) is executed over a polarity reversible timing and the polarity is reversed at the polarity reversible timing, it may well occur that the signal with different polarities is cumulatively added before and after the polarity reversible timing. If a signal with different polarities is cumulatively added, a part or the whole of the received signal is canceled, which causes a problem of degradation in the receiver sensitivity.